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use copyBackwards when copying overlapping items in non-unique.unique concat

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`mem.copy` requires that `dest` has start index `<= src`, but in our
case, `dest` has start index `>= src`. For this, `copyBackwards` should
be used.

See https://github.com/ziglang/zig/blob/master/lib/std/mem.zig#L195-L222.

Closes #4697
This commit is contained in:
Ayaz Hafiz 2022-12-06 12:58:04 -06:00
parent a549176886
commit 4df83d67ea
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1 changed files with 2 additions and 2 deletions
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4
crates/compiler/builtins/bitcode/src/list.zig
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@ -772,7 +772,7 @@ pub fn listConcat(list_a: RocList, list_b: RocList, alignment: u32, element_widt
// This first call must use mem.copy because the slices might overlap.
const byte_count_a = list_a.len() * element_width;
const byte_count_b = list_b.len() * element_width;
mem.copy(u8, source_b[byte_count_a .. byte_count_a + byte_count_b], source_b[0..byte_count_b]);
mem.copyBackwards(u8, source_b[byte_count_a .. byte_count_a + byte_count_b], source_b[0..byte_count_b]);
@memcpy(source_b, source_a, byte_count_a);
// decrement list a.
@ -869,7 +869,7 @@ test "listConcat: non-unique with unique overlapping" {
defer unique.deinit(u8);
var concatted = listConcat(nonUnique, unique, 1, 1);
var wanted = RocList.fromSlice(u8, ([_]u8{ 1, 2, 2, 2 })[0..]);
var wanted = RocList.fromSlice(u8, ([_]u8{ 1, 2, 3, 4 })[0..]);
defer wanted.deinit(u8);
try expect(concatted.eql(wanted));